separation is an attractive alternative to filtration or centrifugation for separating solid catalysts from a liquid phase, Here, core-shell Fe3O4@UiO-66-NH2 nanohybrids with well-defined structures were constructed by dispersing magnets in a dimethylformamide （DMF） solution con- taining two metal-organic framework （MOF） precursors, namely ZrCI4 and 2-aminobenzenetricar- boxylic acid. This method is simpler and more efficient than previously reported step-by-step method in which magnets were consecutively dispersed in DMF solutions each containing one MOF precursor, and the obtained Fe304@UiO-66-NH2 with three assembly cycles has a higher degree of crystallinity and porosiW. The core-shell Fe3O4@UiO-66-NH2 is highly active and selective in Knoevenagel condensations because of the bifunctionality of UiO-66-NH2 and better mass transfer in the nano-sized shells. It also has good recycling stability, and can be recovered magnetically and reused at least four times without significant loss of catalytic activity and framework integrity. The effects of substitution on the reactivity of benzaldehyde and of substrate size were also investigated.
Many efforts have been devoted to the integration of magnetic nanoparticles and metal organic frame- works, which makes it easy and simple to separate the nano-sized metal organic frameworks from liquid phase. Amino-functionalized magnetic metal organic frameworks[Fe3O4@MIL-100（Fe）-NH2] were prepared by a stepwise assembly method followed by post-modification with electron-rich reagent. This magnetic catalyst was characterized by means of X-ray diffraction（XRD）, transmission electron microscopy（TEM）, scanning electron micrnscopy（SEM） and nitrogen adsorption, and tested in Knoevenagel condensation as a base catalyst. The magnetic catalyst exhibits a core-shell structure and can afford a high activity for the Knoevenagel condensation due to its bifunctional property and reduced diffusion limitation. Furthermore, it could be recovered magnetically and recycled three times. Although activity loss was observed in the recycle experiments, it could be reactivated by dispersing in a fresh modifier solution.
